﻿using System;

namespace EasyQuartz.Storage;

public class SnowflakeId
{
    public const long Twepoch = 1288834974657L;

    private const int WorkerIdBits = 5;
    private const int DatacenterIdBits = 5;
    private const int SequenceBits = 12;
    private const long MaxWorkerId = -1L ^ (-1L << WorkerIdBits);
    private const long MaxDatacenterId = -1L ^ (-1L << DatacenterIdBits);

    private const int WorkerIdShift = SequenceBits;
    private const int DatacenterIdShift = SequenceBits + WorkerIdBits;
    public const int TimestampLeftShift = SequenceBits + WorkerIdBits + DatacenterIdBits;
    private const long SequenceMask = -1L ^ (-1L << SequenceBits);

    private static SnowflakeId _snowflakeId;

    private readonly object _lock = new object();
    private static readonly object SLock = new object();
    private long _lastTimestamp = -1L;

    public SnowflakeId(long workerId, long datacenterId, long sequence = 0L)
    {
        WorkerId = workerId;
        DatacenterId = datacenterId;
        Sequence = sequence;

        // sanity check for workerId
        if (workerId > MaxWorkerId || workerId < 0)
            throw new ArgumentException($"worker Id can't be greater than {MaxWorkerId} or less than 0");

        if (datacenterId > MaxDatacenterId || datacenterId < 0)
            throw new ArgumentException($"datacenter Id can't be greater than {MaxDatacenterId} or less than 0");
    }

    public long WorkerId { get; protected set; }
    public long DatacenterId { get; protected set; }

    public long Sequence { get; internal set; }

    public static SnowflakeId Default()
    {
        lock (SLock)
        {
            if (_snowflakeId != null)
            {
                return _snowflakeId;
            }

            var random = new Random();

            if (!int.TryParse(Environment.GetEnvironmentVariable("CAP_WORKERID", EnvironmentVariableTarget.Machine), out var workerId))
            {
                workerId = random.Next((int)MaxWorkerId);
            }

            if (!int.TryParse(Environment.GetEnvironmentVariable("CAP_DATACENTERID", EnvironmentVariableTarget.Machine), out var datacenterId))
            {
                datacenterId = random.Next((int)MaxDatacenterId);
            }

            return _snowflakeId = new SnowflakeId(workerId, datacenterId);
        }
    }

    public virtual long NextId()
    {
        lock (_lock)
        {
            var timestamp = TimeGen();

            if (timestamp < _lastTimestamp)
                throw new Exception(
                    $"InvalidSystemClock: Clock moved backwards, Refusing to generate id for {_lastTimestamp - timestamp} milliseconds");

            if (_lastTimestamp == timestamp)
            {
                Sequence = (Sequence + 1) & SequenceMask;
                if (Sequence == 0) timestamp = TilNextMillis(_lastTimestamp);
            }
            else
            {
                Sequence = 0;
            }

            _lastTimestamp = timestamp;
            var id = ((timestamp - Twepoch) << TimestampLeftShift) |
                     (DatacenterId << DatacenterIdShift) |
                     (WorkerId << WorkerIdShift) | Sequence;

            return id;
        }
    }

    protected virtual long TilNextMillis(long lastTimestamp)
    {
        var timestamp = TimeGen();
        while (timestamp <= lastTimestamp) timestamp = TimeGen();
        return timestamp;
    }

    protected virtual long TimeGen()
    {
        return DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
    }
}